In a mobile communication system, a Base Station (BS) transmits and receives data to and from a number of User Equipments (UEs) within a cell/sector in a radio channel environment. In a multi-carrier or alike system, a BS receives packet traffic from a wired Internet and transmits the received packet traffic to each UE in a predetermined communication scheme. The process of determining a UE to transmit data to and a timing and frequency area in which to transmit the data to the UE by the BS is called downlink scheduling. In addition, the BS receives data from a UE in a predetermined communication scheme, demodulates the received data, and transmits the packet traffic over the wired Internet. The process of determining a UE to transmit uplink data and a timing and frequency band in which the UE is supposed to transmit the data by the BS is called uplink scheduling. In general, a UE in good channel state is scheduled to transmit and receive data for a longer time in more frequency resources.
FIG. 1 is a view referred to for describing time-frequency resource blocks. In the multi-carrier or alike system, communication resources may be divided largely into a time area and a frequency area. The resources may be defined as resource blocks, each resource block including N subcarriers by M subframes or N predetermined time units. N and M may be 1. In FIG. 1, one square represents one resource block and a resource block is defined as a plurality of subcarriers along an axis by a predetermined time unit along another axis. On a downlink, a BS selects a UE according to a predetermined scheduling rule, allocates one or more resource blocks to the selected UE, and transmits data to the UE in the allocated resource blocks. On an uplink, the BS selects a UE according to a predetermined scheduling rule and allocates one or more resource blocks to the selected UE. The UE receives scheduling information indicating that the BS has allocated certain resource blocks to the UE and transmits uplink data in the allocated resources.
In the downlink scheduling scheme, the BS selects time-frequency resource blocks in good channel state based on a Channel Quality Indicator (CQI) representing a downlink channel state, reported from a UE and transmits data using the selected time-frequency resource blocks. Owing to the time-frequency resource blocks in the good channel state, more data can be transmitted in the limited resource blocks, thereby increasing the overall data throughput of the system. Likewise in the uplink scheduling scheme, a BS scheduler measures the reception state of a pilot signal (or a reference signal) transmitted by a UE, selects time-frequency resource blocks in good uplink channel state, allocates the selected resource blocks to the UE so that the UE transmits uplink data in the allocated resources.
The above-described scheduling may be performed on a group basis.
Group Resource Allocation (GRA) is a scheme of allocating resources to a plurality of users within a group in order to reduce the overhead of control messages that a BS transmits to UEs. Since information about resource allocation positions (i.e. resource start offsets and resources sizes), the MCS levels of bursts, etc. that are indicated to UEs in case of resource allocation to individual UEs are compressed and then transmitted to the UEs, control information overhead can be reduced.
FIG. 2 illustrates an exemplary method for allocating group resources using bitmaps.
Referring to FIG. 2, bitmaps may be used to transmit resource allocation information to UEs within a predetermined group. A user bitmap, a first bitmap indicates UEs scheduled at a corresponding time instant in a corresponding group. The respective bits of the user bitmap are mapped to the UEs of the group in a one-to-one correspondence. In FIG. 2, one group may include up to 6 users and if a bit of the user bitmap is set to ‘1’, this implies that a user mapped to the bit is a scheduled user (i.e. a user to which resources have been allocated) for a current frame.
1st, 2nd, 4th and 6th users are scheduled for frame n. When a UE is added to the group, the UE may receive position information indicating its position in the user bitmap from the BS. A resource allocation bitmap represents resource allocation information for the scheduled users. The resource allocation information may contain information about Modulation and Coding Scheme (MCS) levels and allocated resource sizes. In the illustrated case of FIG. 2, resource allocation information for each UE may be represented in 3 bits. Because a total of 4 UEs are scheduled in frame n, the resource allocation bitmap is 12(=3×4) bits long. For frame n+p, 5 UEs are scheduled and thus a 15-bit resource allocation bitmap is formed.
GRA is typically adopted for real-time traffic. When a new packet is generated in an area allocated by GRA, the new packet should be transmitted irrespective of retransmission of a previous packet in order to minimize the delay of the new packet. Therefore, in the case of GRA, multiple Hybrid Automatic Repeat reQuest (HARQ) channels should be used for one connection.
For this purpose, two or more hArq Channel Identifiers (ACIDs) may be used for a connection using GRA. An ACID is an identifier that identifies an HARQ channel and an HARQ channel may mean an HARQ process. ACID information for a multiple-HARQ operation should be shared between a BS and a UE. The simplest way is to include ACID information for each individual UE in scheduling information for GRA, that is, a GRA A-MAP Information Element (IE) as done in a general multiple-HARQ operation. Since the GRA A-MAP IE is transmitted each time group resources are allocated, the inclusion of ACIDs in the GRA A-MAP IE may cause MAP overhead, high MAP overhead especially when more UEs of a group are scheduled through the GRA A-MAP IE.
To avert this problem, when a BS allocates a specific group to a UE, the BS may notify the UE of ACID information by including an initial ACID value and the number of HARQ channels, N_ACID used between the BS and the UE for corresponding GRA in a group allocation message (e.g. a group configuration message or a group configuration A-MAP IE). Each time the UE receives a GRA A-MAP IE, the UE increases the initial ACID value using the received ACID information. If the ACID has been changed N_ACID times, the UE returns to the initial ACID. Thus, the UE can implicitly determine an ACID even though the ACID is not included in the GRA A-MAP IE. In this manner, the overhead of the GRA A-MAP IE can be reduced.
However, in case where the GRA A-MAP IE is lost, there exists a need for defining a method for efficiently recovering a GRA procedure.